1. Field of the Invention
This invention generally relates to a receiving circuit in integrated circuits, and more particularly to an automatic threshold voltage circuit for digital communication and signal converting circuit and method thereof, and more particularly for frequency-shift keying (FSK), amplitude-shift keying (ASK), and on-off keying (OOK) digital communication systems.
2. Description of Related Art
As technology advances, digital communication becomes an indispensable tool for our daily life. Hence, a fast and reliable digital communication system such as FSK/ASK digital communication system is necessary for business or military purpose. In a FSK/ASK digital communication system, the receiving terminal has a comparator to determine if the received bit is logic 1 or 0. One of the comparator input receives the analog signal; the other input receives a threshold voltage. If the voltage of the analog signal is higher than the threshold voltage, the comparator will determine that the received signal is logic 1 (or logic 0). Otherwise, the comparator will determine that the received signal is logic 0 (or logic 1).
Among conventional methods for generating threshold voltage is to use a fixed voltage source as the threshold voltage on chip. FIGS. 1A to 1C show the circuit and the timing of the signal of a conventional analog-to-digital converter where fixed voltage source is provided. Referring to FIG. 1A, the threshold voltage VREF is supplied by a fixed voltage source. The comparator 110 receives the analog signal VIN and compares the analog signal VIN with the threshold voltage VREF. As shown in FIG. 1B, if VIN is higher than VREF, the received analog signal is determined as logic 1 (or logic 0). Otherwise, it is logic 0 (or logic 1). Then the comparator outputs Vout according to the result. Ideally, the threshold voltage VREF is equal to the average value (DC level) of the analog signal VIN. Practically, however, the DC level of the analog signal changes with factors such as temperature, communication ambiance, and parameter migration in manufacturing process. FIG. 1C illustrates the phenomenon, where the analog signal VIN is affected by the above factors so that the DC level of VIN is higher than threshold voltage VREF. Therefore, the comparator 110 will possibly output an incorrect Vout.
To eliminate the drawbacks of the conventional design with fixed threshold voltage, a filter circuit consisting of a resistor and a capacitor (RC filter) was used to filter out the AC component of the analog signal to obtain the DC level of the analog signal as the threshold voltage. Referring to FIG. 2, a conventional analog-to-digital converter that automatically controls threshold voltage using a RC filter is illustrated. The resistor 230 receives the analog signal VIN and is coupled to the capacitor 220 to output the threshold voltage VREF. The other terminal of the capacitor is coupled to ground. The comparator 210 receives and compares the analog signal VIN with the threshold voltage VREF and outputs Vout. In this conventional circuit, although the threshold voltage is generated corresponding to analog signal migration, a large resistor and capacitor is required for low-speed analog signal transmission. For example, if the data rate of the analog signal is 1 Kbps, the required RC constant must be larger than 1 msec; i.e., it requires a 1MΩ resistor and a 1000 pF capacitor, which occupies a large area on the integrated circuit chip. For instance, in a CMOS 0.6 μm process, it requires an chip area of 12 μm×12 μm per 1KΩ and 20 μm×20 μm per 1 pF. Hence, the conventional RC filter is not suitable for an integrated circuit. In addition, once the capacitance and resistance have been determined and integrated into the circuit, the RC time constant is no longer adjustable.